1. Field of the Invention
The present invention relates to an autofocusing technique for an imaging device that converts an optical image, which is captured via an optical system having a focus adjustable function, into a video signal by means of an imaging system.
2. Description of the Related Art
The advance of the digital techniques has boosted the spread of digital video cameras and other imaging devices. The imaging device generally has an autofocusing mechanism that automatically adjusts the position of a focus lens to be focused on a subject. One typical technique adopted for the autofocusing mechanism is hill-climbing control. The hill-climbing control adopts the wobbling technique to identify the focusing direction of a focus lens on a subject and moves the focus lens in the identified focusing direction to obtain an AF evaluation value. The AF evaluation value represents a high frequency component of a luminance signal that is generated by a CCD or another imaging element and is detected by a wave detector. The position of the focus lens giving a peak of the AF evaluation value is specified as a focus position, and the focus lens is controlled to be set in this specified focus position. The wobbling technique minutely oscillates the focus lens at a fixed wobbling amplitude along an optical axis to detect a variation in AF evaluation value and identifies the increasing direction of the AF evaluation value (that is, the hill-climbing direction) as the focusing direction of the focus lens.
The closer aperture of the imaging device leads to the greater depth of the field, so that the wobbling motion at the fixed wobbling amplitude decreases the variation in AF evaluation value. This makes it difficult to accurately detect the hill-climbing direction. In such cases, the hill-climbing control increases the wobbling amplitude with closure of the aperture. The higher AGC gain of the imaging device leads to the lower S/N ratio, so that the wobbling motion at the fixed wobbling amplitude increases the variation in AF evaluation value. This lowers the reliability of the AF evaluation value and thus makes it difficult to accurately detect the hill-climbing direction. In such cases, the hill-climbing control increases the wobbling amplitude with an increase of the AGC gain. The unlimited increase of the wobbling amplitude, however, adversely affects the quality of captured images. When there is a requirement for increasing the wobbling amplitude to or over a specific level, one applicable control technique increases the sampling number of AF evaluation values, instead of the wobbling amplitude, to enhance the detection performance of the hill-climbing direction. A known imaging device with such a control mechanism is disclosed, for example, in JP-A-06-133205.
In inner focus cameras, a compensator lens generally has a function of a correction lens in zoom control for compensating a variation of an imaging surface of a variator lens having a magnification change function, while having a function of a focus lens in focusing control for changing an imaging distance. The optical design of the inner focus camera is ideally determined to eliminate a magnification change due to a position shift of the focus lens. The actual optical design of the inner focus camera is determined to control the magnification change in a practically allowable level according to various restricting conditions including the manufacturing cost and the camera size.
The inner focus camera accordingly has a slight magnification change with a position shift of the focus lens in the wobbling motion. Even the slight magnification change may cause a shake of a captured image and have an adverse effect on the resulting image quality. This phenomenon is especially prominent in the wobbling motion of the focus lens in a position range close to its focus position. There would thus be a demand for an imaging device that ensures the sufficient detection performance of the hill-climbing direction for autofocusing control, while preventing deterioration of the quality of a captured image due to a magnification change in the wobbling motion.